Many devices function by appealing to one or more of the senses. For example, a radio appeals to hearing, a television appeals to hearing and sight, and a merry-go-round appeals to hearing, sight, and feel.
Sound generated by these devices generally reaches the ears by propagating through the air. Yet, devices based on airborne sound transmission suiffer from a number of shortcomings. Conventional speakers produce sound that may be overheard by or disturbing to others. Headphones block the ears and prevent the user from listening to other sounds. Moreover, headphones may be obtrusive and inconvenient.
Recently, a mechanism for transmitting sound to the ears that bypasses the air and external ears has been determined. Through this mechanism, sound waves are transmitted directly to the inner ears, without traveling through air, by conduction through an object to bones in the user's head, from which the sound waves travel through the bones to the ears to be perceived as sound.
The efficacy of devices based on this transmission mechanism is limited by the ability of the object that transmits the sound waves to contact the bone. In some cases, sound waves may be transmitted to the bone by passage through a user's skin and flesh, limiting the volume and clarity of the transmitted sound. In other cases, sound waves may be transmitted to the bone directly, but only after a component of the device is surgically implanted in a user's skull.
These shortcomings may be overcome by transmitting sound waves through an object to the user's mouth, from which the sound waves may be transmitted by the teeth and bones to the user's ears to be perceived as sound. This process is termed denta-mandibular conduction. Because teeth are connected directly to bones in the head, they provide a particularly desirable nonairborne sound conduit to the ears.
Devices based on denta-mandibular sound transmission are disclosed in several U.S. patents. The disclosed devices include a hearing aid, a teeth-held tilt alarm for operators of dangerous equipment, and an underwater communications device for scuba divers. Each of these devices functions only within a user's mouth. The hearing aid is mounted around a user's tooth and amplifies ambient sounds. The teeth-held tilt alarm is clenched between a user's teeth and produces a sound and vibration if the user's head tilts due to drowsiness. The underwater communications device also is clenched between a user's teeth as part of a scuba diving mouthpiece and produces sound that corresponds to signals received from a separate transmitter.
Significantly, these devices suffer from a number of shortcomings. The hearing aid and underwater communications device cannot operate independent of a separate signal source, but instead amplify ambient sounds or reproduce transmitted signals, respectively. The teeth-held tilt alarm produces only a single sound. Moreover, the teeth-held tilt alarm is not volitionally actuated, but instead is actuated if a user involuntarily falls asleep. In addition, all three devices are configured only for denta-mandibular sound transmission or a nonvolitional activity, such as hearing, breathing, or sleeping.